Silicon microfabricated accelerometers are being intensively developed because of the possibility of low cost batch fabrication. Such accelerometers include a seismic or proof mass which is anchored to a silicon substrate by a micromechanical suspension system. In one commercially available chip (ADXL50), the position of the proof mass is detected by a differential capacitance measurement. Using a force-balance configuration, the proof mass deflects in response to acceleration to which the chip is subjected, a sense signal arising from the differential capacitance measurement is amplified and fed back to electrostatically deflect the proof mass and counteract the effect of the acceleration. The output from the accelerometer is the required feedback voltage to counteract the effect of the acceleration.
Tunnel based accelerometers have also been proposed, in which the sense signal is derived from a tunnel current between a sensing tip and a counter electrode. Such a tunnel based sensor has been proposed in a paper entitled "A miniature high sensitivity broad-band accelerometer based on electron tunnelling transducers", authored by Howard K. Rockstad et al and published at the 7th International Conference on Solid State Sensors and Actuators, pages 836 to 839. Such tunnel based accelerometers have a greater sensitivity than capacitive accelerometers, but they require more than one silicon wafer for their manufacture. In the arrangement described in the preceding article by Rockstad et al, three wafers are required to make the accelerometer. Each wafer includes components of the accelerometer which must be carefully aligned when the wafers are secured together. Manufacturing costs of such accelerometers are high because of the need to produce components within three wafers and then to carefully align the wafers during construction of the accelerometer.
It is an object of the present invention to provide an accelerometer which combines high sensitivity and low cost manufacture.